In testing an analog circuit or a mixed-signal circuit, the measurement of analog signals is a big challenge because analog signals are likely to be affected by noise. Further, the accuracy and stability of sampling in the recipient side also directly influences the entire test result. From the viewpoint of signal processing, the analysis of the output test eigenvalues can be accelerated if the outputs are digitized signals. Besides, the digitized signals are less likely to be distorted during transmission. Moreover, the external ATE (Automatic Test Equipment) can also process the digitized signals more efficiently.
There have been sophisticated digital-circuit test methods widely applied to mass-produced chips. However, as great difference exists between different types of analog circuits or mixed-signal circuits, the test technology thereof still has much room to develop. A typical test method for a digital to analog converter (DAC) usually includes the following procedures: the analog output terminal of the tested DAC is connected to the input terminal of a measurement analog to digital converter (ADC); digital control signals are sent to the input terminal of the tested DAC; the measurement ADC digitizes the output voltage signals of the tested DAC and outputs digital signals for analysis. Such a method is a full-digital approach, wherein the generation and analysis of digital signals are undertaken by a digital processing unit. Nevertheless, this method can only apply to a chip simultaneously having ADC and DAC. However, many chips have only DAC. If it is only for testing DAC that high-speed and high-resolution ADC should be built in the chip, the cost of test will be too high, and the chip design will become more complicated.